Self-contained ISOs

Some ISOs have all data needed for boot in the kernel and ramdisk files. They can be booted directly from Grub4Dos without emulation.

?INT13h & Real Mode?

When an OS is in 'real mode', it hooks INT13h BIOS interrupt call and doesn't require emulation. Old or very minimal OSs stay in real mode but new and advanced OSs such as Linux leave real mode. At this point they crash unless properly emulated.

Not sure how accurate I am; I do not fully understand this.

Gujin ISO Modification

According to the website, Gujin can modify .iso files, presumably so that they can be booted directly without emulation. I haven't tried this yet and can't find much info on it.

Physical Workaround

Maybe, instead of a flash drive, a USB SD card reader and some SD cards could be used instead. Still a hassle though.